My feeling is that the stiffness of a bike shoe sole is the first major benefit (MTB/commuter shoes are a little les stiff but absolutely fine). I personally would not be without my SPD pedals on all six bikes (inc a folder) not because of the upstroke but because it keeps my foot correctly positioned and stops it slipping. Getting used to them is like driving a manual car - once used to it you cannot imagine why you ever thought it would be hard....